My experience says the rustproofing does help, but is very dependent on the skill and attitude of the person doing it. I had my 83 done at a Ziebart franchise, in the middle of summer, they were not busy, and the guy took plenty of time.

Yes, they drill holes here and there to get the oil spray into some spots but the little black plastic plugs are not obtrusive.

A drawback to the process I had done is that the stuff turns wax-like after a few days, and is very tough to remove after that. I got it off the exterior body where it had dribbled out from cracks and crevices, but the engine compartment looks ugly.

The main thing is, my near-30-yr-old 240 has very little visible rust after about 25 Canadian winters.
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Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.